Fluorine-containing paints are used as weather resistive paints for exterior and interior coatings of medium- or low-storied buildings, and various techniques have been developed for preparation of the fluorine-containing paints. Though many of the fluorine-containing paints are in the form of organic solvent dispersion, there are problems in the use of organic solvents from safety and environmental protection points of view, and an aqueous dispersion type is preferable. As the methods of preparing an aqueous dispersion of a fluorine-containing polymer, there are known the methods mentioned below.
JP-B-55441/1992 and JP-A-7784/1991 disclose that film forming property and transparency, which cannot be obtained by an emulsion blend, are obtainable by emulsion-polymerization of an ethylenically unsaturated monomer in the presence of particles of a VdF copolymer in an aqueous medium. As one of monomers copolymerizable with VdF in such a VdF copolymer, there is disclosed CTFE, and as examples of the ethylenically unsaturated monomer, there are disclosed ethyl acrylate (EA), methyl methacrylate (MMA) and the like. However the above-mentioned patent publications disclose neither combination of VdF and CTFE nor copolymerizing ratio thereof, and also there is no disclosure as to seed polymerization of EA and MMA in the presence of VdF/CTFE copolymer particles. Also in those patent publications, there is no disclosure as to gloss of the coating, which is a very important property in practical use as paints.
Further though the above-mentioned patent publications state that the coating being excellent in transparency and mechanical strength can be obtained, according to the inventors' actual studies it has been found that the transparency of the obtained coating do not reach the level enough for practical use for clear coating, and thus further investigation is required.
According to the studies made by the present inventors, compatibility of various known VdF copolymers with an acrylic resin is not sufficient even if the compatibility between the resins is intended to be increased by the seed polymerization, and neither a transparent film can be obtained at film formation nor a highly glossy coating can be obtained.
Also JP-A-190745/1989 discloses that an organic solvent solution of a fluorine-containing polymer polymerized in non-aqueous system is converted to aqueous system, and therein a vinyl monomer is then polymerized, and that examples of the polymerizing components of the fluorine-containing copolymer are VdF, CTFE and the like and examples of the vinyl monomer are EA, MMA and the like. However there is no concrete disclosure as to a combination use of VdF and CTFE and a copolymerizing ratio thereof and also as to seed polymerization of EA and MMA in the presence of VdF/CTFE copolymer particles. In addition, the obtained film has problems of being inferior in transparency and insufficient mechanical properties.
Further JP-A-189879/1992 discloses a solution blend of a vinylidene fluoride (VdF)/tetrafluoroethylene (TFE)/chlorotrifluoroethylene (CTFE) terpolymer and an acrylic resin. The VdF/TFE/CTFE terpolymer is compatibilized with the acrylic resin in the presence of an organic solvent, but there is no description as to the compatibilization in the presence of water.
Also JP-B-14392/1982 discloses that a stable mixed emulsion can be obtained by adding a surfactant when emulsion-blending a VdF polymer and an acrylic polymer. However according to the techniques disclosed in that patent publication, no advantage due to compatibilization can be obtained, that is, compatibilized polymer at molecular level like a solvent type cannot be obtained, a transparent film cannot be obtained at film formation and mechanical properties are not sufficient.
The present invention was made to solve the above-mentioned problems, and an object of the present invention is to provide an aqueous dispersion of a fluorine-containing copolymer which is excellent in film forming property, and can give a coating having a high gloss and being excellent in transparency at clear coating, weather resistance and mechanical properties.
The present inventors have found that an aqueous dispersion compatibilized at a molecular level is obtainable by seed polymerization of a specific amount of an acrylic acid ester and/or methacrylic acid ester in an aqueous dispersion containing fluorine-containing copolymer particles (hereinafter may be referred to as "seed particles") having a specified composition comprising 70 to 95% (% by mole, hereinafter the same) of VdF and 5 to 30% of CTFE among the VdF type copolymers in the presence of the fluorine-containing copolymer particles and that the dispersion can achieve the above-mentioned object, and then the present invention has been completed.